1. Field of the Invention
The present invention relates to manufacturing methods of a long size round preform (including clad material) and sheet preform (including clad material) by means of spray deposit.
2. Description of the Related Art
As a forming method of preform (preliminarily shaped material), spray deposit method is conventionally known in which a molten metal is atomized using an inert gas so that the finely broken droplets are rapidly cooled and solidified while they are sprayed to be deposited on a collector. (See, for example, Japanese patent Examined Publication Nos. 54-29985, 56-12220 and Japanese Patent Unexamined Publication No. 64-15264). In comparison to an ordinary casting, the spray deposit method is characterized for example by: formation of uniform microstructure due to rapid cooling and solidification; free from macrosegregation; and excellent workability and forgeability.
Further, production methods with simplified rolling process are generally known in production of sheet preform (strip), i.e., such as single roll system, roll belt system, twin roll system, and twin belt system are known as special types of continuous casting facility where melting .fwdarw. continuous casting .fwdarw. rolling are consistently performed.
When a long size round preform is formed by a conventional spray deposit method, less porosity is achieved and a preform with higher density is obtained and at the same time productivity is higher comparing to thermal spraying in which a laminated coat is formed by spraying particle groups of a molten metal to the surface of a clad material.
If such rapid cooling and solidification are excessively performed, however, porosity appears and a high density preform is hardly obtained. Presumably, the cause of such porosity is that, if cooling by gas is excessively performed, the ratio of solid phase particles to the particles to be cumulated on the preform is increased and the number of liquid phase and semi-liquid particles that are to fill the gap between the solid phase particles is reduced whereby the gap between the solid phase particles remains as it is to cause porosity. Further it is presumed that, when the temperature of a particle is relatively low, the particle is less likely to be flattened at the time of its collision and a part is formed which is hidden from the particles that will come next. Such hidden part is filled if those particles coming next are at a sufficiently high temperature to have proper liquid area. But porosity appears if particles temperature is too low or if the density of the particles is relatively low, because the particles coming next are unable to completely fill the gap between former particles.
Although, as described, the cause of occurrence of porosity in a preform is presumably related to temperature, its cause is not yet clear-cut. For example, when a round preform is manufactured, porous layer often appears in the interface of a tube-forming collector. From this analysis, it is assumed that the particles contacting the collector are lowered in temperature and porosity is caused due to the reason as described above.
Thus, preheating of the collector is to be considered. Application of such as high-frequency heating or plasma heating as the method of preheating, however, causes discontinuity of preheating (i.e. temperature difference in the longitudinal direction) in the case of a tube-like long size article and the occurrence of local porosity still remains as a problem.
On the other hand, use of the above-described single-roll type continuous casting (single roll type strip caster) as the method for obtaining a sheet preform is simple in structure and at the same time is capable of providing a relatively large cooling area. It thus has an advantage of higher productivity for example comparing to a twin roll type strip caster. It is difficult, however, to control the thickness, and, since the cooling rate at the roll side and the cooling rate at surface side of the strip are different, defects are easily caused at the superficial portion of the strip.
Moreover, there is also a problem that its productivity is significantly lower comparing to an ordinary continuous casting facility which produces cast pieces such as of slab or billet.
Further, spray deposit method is also known as a manufacturing method of sheet preform. With the spray deposit method, however, porosity appears and a preform having high density cannot be obtained when rapid cooling and solidification are excessively performed, as is the case as described in forming a long size round preform.
Thus, the present inventor has already proposed a manufacturing method of composite metal material using spray deposit method, as described in the above-mentioned Japanese Patent Unexamined Publication No. 64-15264. The manufacturing method is thereby disclosed in which a separately prepared molten metal to be used for preheating is caused to flow down on a plate base metal to form a coating layer on the base metal prior to spray deposit for a different metal and spray flow is then cumulated on this coating layer for the different metal. In such manufacturing method, a coating layer of preheating molten metal is interposed to increase the joining force between the different metal and the base material. Thus, while formation of a preform integrally incorporated into the base material is possible, there is a problem that it is impossible to continuously obtain a formed product having a plate preform layer.